Has anyone attempted to train an AI to learn all languages?
There are approaches in machine translation that try to capture this kind of synergy between languages. The idea is that if you train your architecture to be able to translate English-Japanese, Japanese-English, Korean-English, English-Korean it will also be able to translate from Japanese to Korean without ever having seen a single such training example. Here, you can read about this so-called zero-shot translation.

It is also possible to train wordvectors on several languages at once, which might give you better wordvectors for a language with few training examples. 

Of course for true language understanding you have to solve the grounding problem and I'm not sure using several language will help with that.